Radio-frequency dielectric heater with constant heating rate control



June 14, 1949. F. G. ALBIN 2,473,188

RADIO FREQUENCY DIELECTRIC HEATER WITH CONSTANT HEATING RATE CONTROLFiled June 1'7, 1944 4. Cfnput ATTORNEY.

Patented June 14, 1949 RADIO-FREQUENCY DIELECTRIC HEATER WITH CONSTANTHEATING RATE CON- TROL I Frederick- A-lbi'n, Los Angeles, Calif.,assignor'to Radio Corporation otzAmerica, a corporation of DelawareApplication June '17, 1944, Serial No. 540,767

5Claims. (CL 250-86) This invention relates to high or radio frequencygenerators for industrial heating purposes, and particularly to such ageneratorwhic'h is completely automatic in itsoperation from thestandpointof being independent of size or shape of the material to beheated within predetermined "limits.

The use of radioirequency for inducing heat into material for various'purposes is well-known "in the art. For instance, radio frequencyheating is used for setting glues in ply-woodmanufacture and to heatpreforms i-n the plastic'm'olding industry. The present invention isparticularly applicable to the use of radio frequency for heatingsuccessively a number of plastic preforms of different sizes and shapes,the system providing a, constantpredetermined heating rate regardless ofthearea or thickness of the preform to be heated. In the past, it hasbeen necessary to vary the-output voltage or current by -"tuning' theload circuit. This required a complex procedure and the manipulation ofa luralityof controls for obtaining resonance and impedancetransformation of the desired degree in order to obtain the desired rateof heating of the material. Because of this complexity, any automaticcontrol by load variations was heretofore not considered feasible.

In the present invention, it is onlynecessary to pro-establishan-opt'imu-m heatingrate in watts per cubic unit of the particular typeof material after which theautcmatic controls maintain this heating rateindependently of the area or the thickness of the specimens and of theelectrical characteristics of the material whichc'hange during theheating period.

In my copendingapplicatlon, Ser'; No. 560,465, filed October 26,1944,]?atent No. 2,442,451, dated June 1, i948, the mechanical featuresresiding inthe size, shape, and arrangement of electrodes and in thesafety features of the unit are disclosed in detailand claimed, thepresent applicati'on being directed to and claiming the circuit elementswhich provide the system with a com stantheatingrate andautomaticcircuitcontrols.

An object-of the invention, therefore; isto'facil itate' the operationofa radio frequency dielectric heating system. Another object of theinvention "is to provide animprovedmethod of supplying radiofrequencyheat'to material spcci-mens-difierlng in size and shape.

.Aiurther objectoi the invention is to provide an improvedoscillatorsgener ator system. which is completelyautomatic in itsoperation.

A further object of the invention is to provide an oscillator adapted toprovide automatically a constant heating rate for material specimens ofdifferent sizes and shapes.

A still further object of "the inv'e'en-tion' is to provide anoscillator-generator system which is automatically adjusted with respectto voltage and power output in accordance with the size and shape of thespecimens of material being heated.

A still further object of the invention is to provide an industrialheating generator with automatic output controls and automatic safetycontrols operative when the output exceeds a predetermined value or theperiod of automatic aerjustment exceeds a predetermined time.

Although the novel features which are believed to be characteristic ofthis invention will be pointed out with particularity in the appendedclaims, the manner of its organization and the mode of its operationwill be better understood by referring to the following description readin conjunction with the accompanying drawing forming a part hereof, inwhich the single figure is a schematic diagram of a high frequencygenerator embodying the invention.

In general, the circuit shown in the diagram is an oscillator of theself exciting type employing two oscillator tubes 5 and 6 connected inparallel, the filaments of which are supplied with alter nating heatingcurrent over a transformer 50, while the radio frequency circuitsthrough the filaments are connectedover respective condensers 8, 9, l0,and H. The plates or anodes of the oscillator tubes 5" and 6 aresupplied with direct current potential over a reactor l3, by-passedbycondenser l4, and conductor Hi from the high side of rectifier tubes I8,I 9, 21!, and 2] connected in a bridge arrangement, the remainder of theplate circuit being over filter" choke-25 relay coil 26 and apotentiometer resistor '21, the current flowing in the latter elementbeing proportional to the watts taken by the load when the D. voltageacross c'onde'nserbllis constant; which it is.

Power is supplied to the generator from any suitable single phasesource, such as a 230 voltSll or 60 cycle line connected at plug 3 2.One of the three plug conductors is grounded to the frame at 33, and theother twoconductors are connected to a main switch 31 overloadthermal-tripping v resistors 35, safety switches 36 on the doors-oi thecabinet containing the generator, fuses 3 1 to a voltage regulator 38,an indicating on lamp 39, shunted by a condenser 40", a cooling blow'erj42., and a pair of normally open contactsls and 46 of a relay 48, Thus,when switch 34 is closed, power is supplied to the voltage regulator andthe blower 42 and the lamp 39 are energized.

From the voltage regulator, one circuit feeds the primary of thetransformer 58 for supplying heating current to the filaments of tubesand 6, the primary of transformer 5| for heating the filaments ofrectifier tubes I8, I9, 28, and 2I, and the primary of transformer 52for heating the filament of a single rectifier tube 53. The othercircuit from the voltage regulator 38 is connected over fuses 55 to atime delay relay 56 of any suitable type for providing a delay ofapproximately 30 seconds to allow for proper stabilization of thevaporization and condensation of the mercury in the rectifier tubesbefore the plate voltage is impressed thereon. The next element in thiscircuit is a safety switch 51 which is closed only when the safety cageor guard over the work electrodes is in position to protect theoperator, as

described in detail in my above-mentioned copending application.Following switch 51 are contacts 26' of relay coil 26, which are openedwhen the plate current of the oscillator is abnormally high. Next comesan interval timing mechanism 58 for energizing a pair of infrared lamps59 and for energizing an electric door latch 63 for releasing the safetycage and disconnecting the high voltage from the oscillator after apredetermined time interval. This mechanism is described in detail in myabove-mentioned co-pending application.

The next element in this circuit is a normally closed manual stop switch68, the circuit then branching to a normally open contact 6| on relay48, and the coil of relay 48, and back over conductor 62 to the voltageregulator 88. The other branch goes to a normally open start switch 64and then to the coil of relay 48, and back to regulator 38. The closingof start switch 64 will supply energy to the coil of relay 48 which willcause the relay to close its contacts 45, 46, and Bi. The closing ofcontact 6I will now supply energy to the coil of relay 48 over closedstop switch 68, thus permitting the start switch 64 to be opened whilemaintaining the energizing circuit to the coil of relay 48 is closed.Thus, relay 48 is a holding relay, and when once energized will connectalternating current energy to the plates of the rectifier tubes overcontacts 45 and 46 through transformer 66. Simultaneously, an indicatorlamp 61, shunted by a condenser 68, is energized to show that power isbeing supplied to the rectifier transformer 66, this indicator lampbeing associated with a resistor 69 to lessen the current through lamp61.

Connected across the second circuit from regulator 38, when a manualswitch 13 and a relay contact 14 are closed, is a coil 18 having anarmature and a pair of contacts H. The contact 14 is normally openexcept upon energization of its coil 15. When the coil 18 isde-energized, the right-hand contact 1I will be made to supply highpower to the rectifier tubes I8, I9, 28, and 2! over transformer 66, andwhen the coil 18 is energized, the left-hand contact will be made tosupply less energy to the plates of the rectifier tubes. Thus, withcontact 14 closed, it is only necessary to close the switch 13 to reducethe voltage on the rectifier tubes. Similarly, if relay coil 15 isenergized when switch 13 is closed, coil 18 will be energized and lowpower will be impressed on the rectifier tubes. This is an overloadsafety feature for the oscillator, and will be explained hereinafter.When a low power condition exists by theenergization of coil 15, acircuit is closed through an indicator lamp 11, with a shuntingcondenser 18 and a protective resistance 19 to indicate that the systemis operating on low power. The relay 1415 has a time delay ofapproximately three seconds to prevent its operation and the switch overto low power if overloads exist for shorter periods than this, suchoverload periods being within the safety limits of the system.

Referring now to the filament heating circuit for the tubes 5 and 6, asmentioned above, power for these tubes is supplied over transformer 50.The midtaps of the secondary windings of the transformer 58 haveconnected thereto a resistance network of resistors 8I, 82, 83, 84, and85. These resistors are connected between ground and the filaments ofthe tubes 5 and 6, resistors being many times the largest value in thenetwork. The voltage drop across these resistors serves as a portion ofthe grid bias potential for the tubes 5 and 6.

As mentioned above, the tubes 5 and 6 are connected in parallel and thegrid leak circuit includes resistors 81, 88, and 89 in series. Ablocking capacitor is provided at I84. Also included in parallel fromthe grids of tubes 5 and 6, are a resistor 9I, shunted by a reactor 92,and a resistor 93 shunted by a reactor 94, these combinations beingparasitic oscillation suppressors.

Connected across resistors 8|, 83, and 84 are a plurality of switches96, 91, and 98, in association with a milliammeter I88 shunted bycondenser I8 The resistor 84 supplies a potential which is conductedthrough the normally closed contacts of switches 96, 91, and 98 to themeter I88, where the indication is directly proportional to the currentthrough the resistor 84 and consequently proportional to the combinedspace currents of tubes 5 and 8. When only the switch 91 is actuated,only the potential across resistor 8| will be connected to the meter I88and only the space current of tube 5 will be indicated on the meter I88.Resistor 83 is similarly connected through switch 96 which, whenactuated, will permit the meter I88 to indicate only the space currentof tube 6. Resistor 82 is a balancing potentiometer and provides amanual control of the relative grid bias potentials on the two tubes 5and 6, enabling the space currents thereof to be equalized. Switch 98 isconnected across grid leak resistor 89, and when actuated, serves toindicate the current in the grid leak circuit. This circuit arrangementthereby normally indicates the total space current of the oscillator,but will indicate the space current of each tube individually, or a gridleak current on the same meter by simply pressing the appropriate switchbutton.

The oscillating portion of the circuit includes a plate blockingcapacitor I85, a reactor I86, a reactor I81, a three-electrode variablecondenser assembly H8, and a two-electrode variable condenser III,together with the capacitance of the load I I4 placed between applicatorelectrodes which are connected to the central variable electrode ofcondenser assembly I I8 and its variable bottom electrode which is alsoconnected to the grounded top electrode of condenser III. Forconvenience the upper section of condenser assembly II8 has beendesignated capacitor III), while the lower section has been designatedas capacitor H8. A reactor H6 is connected from inductor I86 to groundto bleed off the static charge from the tank circuit. A tuned circuit isshown consisting of an inductance H8, 2. capacitor I I9, and anindicating lamp I28, in combination, which serves as a wave indicator toindi- :tete when "the tank circuit is properly tuned tompredeterminedlirequency. iInductance l l8zmay becoupled toeither:inductance .I:ll6or.inductance illtLlamp m bGingEPI'GIGI'HbIYaM-KESfillEd lamp which will .iIluminate when "the oscillator freguency:is in .resonance with the =wave indicator circuit. In thisarrangementreactor1.05 in com- :hination with reactor [.07 constitutes theinductancelof the basic oscillator circuit, while the combination-ofcondenser assemblyd M ,condenser J .H, and the capacitance of the: load.114 together with certain stray capa'oitances of :the connected tubecirouits constitute the xtank .circuit capacdtancesofthe-basica'oscillator circuit. Condenser Hal .xi'S manuallyadjustable, :and when once-set, rem'ams rfixed. As mentioned above,condenser 1:18 :ds .:.made up of -.three plates, the lower and center,plates of which are automatically adjustable in accordance withvariations in area and thickness, respectively, of the material to befheated as will now be explained. It may .be observed at this point thatcondenser assembly I I is-ressentially .two variable capacitorselectrically connected in series and 1 so arranged that as thecapaoityofaone-(HO increases the other ('I l0) decreases, and viceversa. In addition, one of these-capacitors (ll fl lin this case) isalsoindep'en'dently adjustable.

. l:Bhovvn: diagrammaticallyare :motors l 3.0 and I3 I which. are:energ-ized overconductors I32 and 133 from the power line when a switch135 is closed. The-:motorsarezreversible, ,motor :I-3B rotating in onedirection whenarmature 13:! .of relay I38 is moved to thelleft, androtating: in the other direction when'the: armature *l-3l.-isrmoved tothe right. Similarly, for motor :l=3l, which rotates in one directionwhen-armature M0 oi-relay MI is moved to the 1 left, and in the otherdirection *when the armature M0 is moved to-the right. The .motor 1- 30,through a "worm andpinion gear arrangement I43, rotatesi'agpointer 1'44on a dial I45 calibrated-imarca variations, rotation of :the pointershaft simultaneously moving .the lower plate of condenser l lll towardandaway from the central .platein accordance with thedirection ofrotation of the .motor 130. Similarly, the motor l-3I :is gearedthroughigears lflato the shaft of a pointer MBion a dial MEI-calibratedin thickness'variations and'to thecenter,plate orelectrod'e of condenserM0, the central electrode being .thereby .adjustable between the upper:and'lower electrodes :in accordance with the direction :of rotation Ofthe motor 13L Between-the dials l45 and 'l49,-.are two potentiometers t!and I52, potentiometer 1! 5| having, its contact simultaneouslymovablewith the shaft ofipointer l'df L'and the'con-tact'ofpotentiometer [52 having its contact simultaneously movable withthe-shaft of pointer 148. The original adjustments of these elements aresuch that when thecontactofpOtentiometer l'5l isat its extreme.bottomposition, the pointer on dial I45 indicates z'ero area. Thecontactof potentiometer I52 is also at its extreme bottom position whenthe pointer I48 indicates zerothickness .on the dial mentioned above,the :oscillator circuit .is of the type wherein the .voltage acrosscapacitancel H is the grid voltage, and'the voltage across capacitancesH Mama 1- Illtogether with theload capacitance efiectively .in paralleliWith I T0", "is the highirequency plate voltage. Furthermore, if theztotalcf athe capacitances H0 and I [4 is constant, the-grid voltage is:constant, and any change in the total capacitancew'of thesetelementsiii will cause a change :in the high :frequency .egrid voltage, andconsequently will cause a corresponding change in direct grid current:fiowing through resistors :81, 88, and 89. Now, if the loadcapacitance'changes, adiustmentcfcapaoitor H 0' may .biesmadeto: returnthe total capacitance to its predetermined value and to thepredetermined ztunedv-frequenoy of the tank circuit.

2 m'changein :the total capacitance is caused by placing between theapplicator electrodes specimens of greater or less than normal area, sothat a change in the grid voltage T'ELCITOSS capacitor .Hril results,and-consequently-there is:a change in the gridcurrent flowing inresistor I88. Thischange in grid current causes the motor 13.0 to beactu-ated' in either of its two directions as will now be explained.

'The systemis soaarranged thatit will :automati'cal'ly .retune itselfwhen diiierence in :the areas of the work specimens cause a change inthe total capacitance of the elements H0 and H4. The voltage developedacross resistor 88 is impressed overcconduotors I55 on the left-handcoil 1530f the double polarized :relay [38. To provide a reference point:based on optimum "operating conditions, the output :potential ofrectifier "53 is impressed across the right-"hand coil 154 of relay 138over conductors 156. The output'of this-auxiliary rectifier 5:3 .is usedas a reference since *itderi-ves its anode voltagefromzthe-same .primarysource as the high voltage transformerlili. The direct current voltage.from the rectifier '53 will occur across capacitor .151. Thus, when theoscillator is :set pior optimum routput conditions for a given specimen,the .contactionpotentiometer all'isad justedto provide a balance betweenthe two-coils lfisand I54 of relay I38 so that its armature 1 31 is 'inneutral position.

If, thenya largerispecimen is placed between the applicator electrodes,the increase in load capacitance initiallyresults in a decrease in thecombined.reactance ofztheiload and condenser H0 combination. An increasein current through condenser Hzl new results whichcauses an increase inpotential'thereacross. Suc'lr'an increase in grid excitation results .ina corresponding ;increasein idire'ct'gridacurrent which causes acorresponding lincrease .in voltage across resistor "88. Thiswill 'causea differential in the flux produced by the two'windings I153 and J54 ofthe relay 138 and will swing the armatureto the'left to rotatethemotortto decrease-the capacitance of the condenser -'l I ll. .Bydecreasing thiscapacitance, the voltage across resistor58'8will-decrease and bring the 'diiierentialfluxzfrom the two windings ofthe relay 1 38 again to zero, and the armature 131 will again. assume-aneutral position, stopping'the motor. The .controlwill operate in anopposite direction if a smaller sample is placed in the applicator.

:As another-example, assume that thegrid current is abovenormalrel-ative to the plate voltage caused by the load :capacitancesbeing too high ftorthe setting- "ofthe pointer M4 on the dial I45.

' Inrthatcase, the-armature l3! is actuated to the left, which willrotatethe motor I30 in a direction which will cause the pointer 1-44 toincrease, thereby restoring the normal over-all tank capacitance, tankcurrent, grid current,=and frequency. Areduction of grid currentbelow-normal causes theoppos'ite action to 'takeplace. Asthe pointer M4is moved together withthe bottom electrode of condenser ll-ll thecontact of potentiometer- LSI is also moved accordingly.

xSimult'aneous :with 'the closing of the motor contacts by armature I31,a'second pair of contacts is also closed thereby supplying a potentialto the coil 15 over conductor I59 and one of conductors I33. Asmentioned above, this relay has a delay of from two to three secondsafter energization, so that if the regulatory period persists for overthis length of time, the contact I4 is made, which energizes coil I andplaces the power voltage across the full primary of transformer 80, thusreducing the power to the oscillator and thereby safeguarding the tubesand other circuit components against damage. It will be noted that asimultaneous reduction of potential is made on the two coils I53 and I54of relay I38, thereby enabling the system to continue to retune itselfand reach a balance on low power as well as on high power. When thesystem has completed its regulation, the armature I31 will assume itsneutral position, breaking the circuit of coil 15 and thus restoring theoscillator to the high power condition. From the above, therefore, themotor relay combination I3Il-I 38 provides an automatic control forspecimens of various areas together with a safety feature which reducesthe power supplied to the oscillator if the regulatory period becomestoo long for the safety of the apparatus.

The next automatic adjustment which can be accomplished with the presentsystem is to compensate for specimens of various thicknessesindependently of their areas. The generator is adjusted to provide anoptimum heating rate in watts per cubic unit of material limited by thecapacity of the generator. The heating rate is reflected in and isproportional to the current through resistor 21, which current is theplate current of the tubes 5 and 6. Thus, a potential proportional tothe plate current flowing through resistor 2'! is impressed on a rigidlymounted reference coil I60 of relay MI. The relay MI is also providedwith three additional coils, namely, IBI, I62, and. IE3. The movablecoil I6I on armature I40 is excited by direct current proportional tothe plate voltage similarly to coil I54 of relay I38 which causes atraction force to be exerted between these coils I60 and I6I, whichforce is proportional to the input power of the oscillator, and assuminga constant efficiency, is proportional to the output power or the powerinto the load II4. as the frequency, current, and resistance of thecircuit are constant.

The next coil I62 of relay I4I which is also movable with armature I40is connected over conductors I65 to area potentiometer I5I, whilerigidly mounted coil IE3 is connected over one of conductors I65 and aconductor I68 to thickness potentiometer I52. As mentioned above, as theshaft of pointer I 44 and the contact of potentiometer I5I are adjustedin accordance with the area of the work specimen, the voltage across thepotentiometer I5I impressed on coil I62, is proportional to the area ofthe load. The voltage across potentiometer I52, according to the settingof pointer I t8 and as impressed on the coil I63, is adapted to vary inaccordance with the thickness of the load. Thus, a traction force existsbetween coils I52 and I63, which force is proportional to the product ofthe area and thickness, which product equals the volume of the workspecimen. The coils of relay I4I are thus so related that the tractionforce resulting from the product of area and thickness of the specimenis opposite to the traction force resulting from the product of inputvoltage and current, the direction of movement of the armature I40 beingin accordance with the The circuit efficiency is constant, inasmuch 8result of the differenteial products. "Thus, when a predetermined ratioof these two products, which is the ratio of watts per cubic unit, orheating rate, is established, the armature I40 assumes a neutralposition, this neutral position being obtainable by the adjustment ofpotentiometer 21.

The automatic thickness control operates as follows. Assuming a casewhere the work specimen is thicker than the normal specimen for whichthe heating rate was initially adjusted as indicated on the thicknessdial I49, the load current would then be less than normal because theeffective series resistance of the particular specimen II4 would begreater than normal, and it would thus absorb less power from theoscillator. Now, the force resulting from the volume product, that is,the traction force between coil I62 and I63 would preponderate over thetraction force resulting from the power product, that is, between coilsI60 and IBI, the differential causing the armature I40 to be actuated tothe left, closing the respective contacts. The motor I3I will thenrotate to move the thickness control to a new setting of a greater valuein accordance with the thickness of the specimen, and the centerelectrode of condenser I I0 will be adjusted to increase the loadpotential, current, and power, and thus the input power to theoscillator until the traction forces on relay I4I are balanced. Thereverse action takes place if the work specimen is of less thicknessthan rated.

The same safety feature is included in the thickness control as in thearea control, that is, if the regulatory period for adjustment forthickness is longer than that required for safe operation, the coil I5is energized over conductors I33 and I at the same instant that themotor I3I is energized, and should the regulatory period extend overdelay setting of the coil "I5, the power to the oscillator will bereduced and again restored after regulation has been completed.

' The above automatic features provide an inherent stable operatingcondition since any forces or motions causing a displacement from thenormal conditions produces counteracting forces or motions in theopposite direction to restore a balanced condition predetermined by: themanual setting of the heating rate control 27. That is, the area controlis based on variations in total capacitance of condenser H0 and specimenH4, which affects the grid current, resulting in an adjustment of thebottom electrode of the condenser Hu of heating is directly proportionalto the voltage per inch of thickness of the specimen, changes in theproducts of the area and thickness are made to balance the output powerto the specimen. Thus, should the output power vary,

caused by variations in thickness of the speci' men, or by changes ofelectrical properties of the material which usually occur because of theternperature change, these changes will be reflected in the differentialbetween output power and and an automatic adjustment will be madeaccordingly. Thus, it is only necessary for an operator to predeterminethe rate of heating desired by adjustments of potentiometers 21 and 88after which the system will provide the same' heating rate for allspecimens regardless of their shape, size, or electrical propertieswithin the capacity of the system.

Similarly, since the rate If :the electrical properties, such as "lossfacitor or power factor" vary, either as a result of heating, or as aresult of differences between :specimens treated, automatic readjustmentof 'powerwill result, even though the dial indication of ithicknessmaynot then checkwi'thlthe actual "measured thickness of the material. Inthis sense, the dial calibrations :are not :necessarily preciseunlessseveral scales are provided on one dial to provide for variations ofdielectric .constant, power factor, and loss factor of the material. Thearea and thickness dials, .howeven, are particularly useful .for .apreliminary setting of "the electrodes of condenser-assembly .Illl.

One preferred form of generator in .accordance with the inventionutilized type RCA 833A tubes for the oscillator at '5 and "6., type RCA8.00.8 rectifier tubes at 1'8, 119, 210,, and '21., and a type RCA '83rectifier for "tube 53. The coupling capacitor 105 had 2'00'rnicrom'icrofarads and .capacitor I 16 Thad 0.002 microfarads. Theplate choke l3 and the bleeding reactor I16 were .10 micro'henries each,while the inductance of .reactors 92 and .94 were .05 microhenriesshunted by resistors of 100 ohms each. The resistance of resistors 81,8B,.and 8.9 were Zoo-ohms, 500 ohms, and 5 ohms, respectively, theresistance of resistors 8i 183, and M were 1 ohm eachnthe-resistance ofresistor 82 was 100 ohms, and the resistanceof resistor 85 was 1-25ohms. The filter inductor 25 had an inductance of 5 .henries, while thepotentiometer resistor 21 had a resistance of .250 ohms. The highvoltage impressed upon the rectifier tubeswit-h switc'h'll in theright-hand or high power positionwas approximately 3.650

volts, while the voltage impressed on these tubes with the switch '71 inits left-hand or low power position was 1875 volts. The interval timer518 is well-known as a Controlflex' timer. "The other elements of thesystem are Well-known, although it might be stated -that the resistanceof each coil of the double polarized :relay 13 8 and relay .MI "wasapproximately 500 ohms.

The infrared lamps are for heating the load electrodes during the"heating cycle to supplement the heating by radio .frequency power, andthe lamps are energized when the safety cage over the electrodes ismoved to closed position. The closing of the safety cage alsoclosesswitch 51 so that the relay 48 can be energized, the cage being heldclosed by electric door latch 63 which is tripped by timer 58 after apredetermined heating period.

In a preferred form, the oscillator frequency was approximately 27.4megacycles, obtained when inductor I06 had two microhenries, inductorI01, three microhenries, condenser HI, 200 micromicrofarads, and thetotal capacitance of condenser assembly ll!) together with the load was50 micromicrofarads. This permitted the capacitance of the load to varyfrom zero to 50 micromicrofarads, it being noted that the two sectionsof the condenser assembly I ll! are in series, while the lower sectionis connected in parallel with the load capacitance so that if the uppersection Hu of condenser assembly I I0 had an infinite capacitance andthe load had zero capacitance, the lower section Hu has 50micromicrofarads. Now, if the upper section had 50 micromicrofarads andthe load capacitance was zero, the lower section would then haveinfinite capacitance. Another combinatlon to obthin 50 micromicrofaradsover-all capacitance would be for the upper and lower sections to each.

i0 Zhave 1B0 -microniicroiarads and the load zero capacitance.

The upper section of condenser assembly Ill] is in series with the.circuit from the tank circuit to the load and is varied by ;the motion.of the central electrode relative to the upper and lower electrodes.The radio frequency current through this uppersection is constant sincethe tank vfrequency :and voltage are constant and the com- .binedparallel capacitance of the ,load and the lower section of condenserassembly llll is held constant. Thus, the potential across the uppersection of the condenser assembly H0 is inverse'ly proportional to thecapacitance. Furthermore, since the same current flows through the lower'sec'tionand the load in parallel as through the upper section, the,potential across the lower section is inversely proportional to thecombined capacitance oi the load and the lower section. The variablecondenser assembly Hi1 thus serves as a voltage divider, dividing thetotal voltage across its outer electrodes into two portions iniinverserelationship to the capacitance of "the two sections, 'the lowercapacitance including that of :the load.

The variation .of capacitance "in this design is accomplishedby avariation in the spacing between the electrodes of condenser assembly II0 so that the potential gradient will be constant, movement of thecentral electrode bearing a linear relationship to the change .inpotential across the upper section. However, since the lower section "isconnected in parallel with the load, any movement of the lower electrodeto maintain the over-all capacitance constant, makes it "necessary thatthe capacitance of the lower section vary in a .manner so that therelationship between this combination and the area dial indications andvoltage variations across potentiometer 151 'be hyperbolic.Mathematically stated, this relationship is:

.s K -K where S is the spacing between lower section electrodes, .A isthe area dial indications, .and K1,, K2, .and K4 are constantsdetermined by dielectric materials, area of the electrodes, and over-allcapacitance.

'Thus, difiering from the linear relationship between the thickness dialindications or voltage variations across potentiometer 152 and movementof "the central electrode, movement of the lower electrode is not linearwith respect to the area dial indications or the voltage variationacross the potentiometer it! but in accordance with the aboverelationship. This hyperbolic relationship may be accomplished in anywell-known manner such as variably winding the potentiometer l5! in ataper or by providing a hyperbolic mechanical link or cam mechanismbetween the shaft of pointe M4 directly connected to a uniformly woundor linear potentiometer and the lower electrode of condenser l I0 Themotor I30 may be connected to the shaft of pointer I44 as shown or toany other moving part of the mechanism as long as the movement betweenthe lower electrode of condenser H0 and the voltage variation acrosspotentiometer I5! is related as stated above.

I claim as my invention:

1. An electrical generatorsystem for heating a load with current fromsaid system comprising a vacuum tube having an anode, grid, and acathode, a feedback circuit between said anode and said grid, a tunedcircuit connected to said anode, grid, and cathode, said tuned circuitincluding inductance and capacitance determined by the capacitance of amultiple-electrode variable condenser having two adjustable electrodesand the capacitance of said load connected to said system, thecapacitance of said variable condenser and said load capacitancedetermining the feedback current in said feedback circuit connected tothe grid of said tube, means for supplying said generator with directcurrent for the anode of said tube, and plural motor means for varyingthe adjustment of the multiple electrodes of said variable condenser tomaintain the total capacitance of said variable condenser and said loadat a substantially constant value, said last-mentioned means including afirst motor connected to one adjustable electrode of said variablecondenser, a second motor connected to another adjustable electrode ofsaid variable condenser, a power supply for said motors, and means forrotating one of said motors in one of two directions according to thevariations in grid current caused by changes in the capacity of saidload.

2. An electrical generator in accordance with claim 1 which includes arectifier having an output current proportional to the D. C. anodevoltage applied to said tube, said rectifier output current providing areference to which said variations of grid current are compared tocontrol the direction of rotation of said one of said motors.

3. An electrical generator system for heating a load with current fromsaid system comprising a vacuum tube having an anode, cathode, and agrid, a tuned circuit connected to said anode, cathode, and grid, saidtuned circuit including at least one inductor and a variable condenserhaving multiple adjustable electrodes, the inductance of said inductorand the capacitance of said variable condenser and capacitance of saidload connected to said generator determining the frequency at which saidgenerator oscillates, plural motor means for automatically adjusting theelectrodes of said variable condenser for varying the capacitancethereof in accordance with vari ations in said load capacitance, and apolarized relay for utlizing the unbalance between a force proportionalto the plate voltage of said generator, and a force proportional to thegrid current of said enerator, said relay means controlling thedirection of rotation of one of said motor means in accordance with thedifierential between said forces for varying the adjustment of one ofsaid 12 electrodes of said variable condenser in accordance with thevariations in capacitance of said load.

4. An electrical generator system in accord.- ance with claim 3 in whichsaid system includes additional relay means for obtaining a balancebetween forces proportional to the electrical power output of saidgenerator and forces proportional to the volume of said load, saidadditional relay means controlling the direction of rotation of anotherof said motor means in ac: cordance with the differential between saidlast mentioned forces for varying the adjustment of another of saidelectrodes of said variable condenser in accordance with the powerrequirements of said load.

5. An electrical generator system in accordance with claim 3, saidsystem including means for removing the power from the anode of saidtube when the anode current exceeds a predetermined value, and means forreducing the anode voltag on said tube when the period of adjustment ofsaid variable condenser exceeds a predetermined length of time.

FREDERICK G. ALBIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Great Britain Dec. 2, 1935 OTHER REFERENCESHoyler, Electronics, August 1943, page 92, (Reprint in Division 60.)

